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tpolecat/gist:7401433

Last active August 29, 2020 08:00
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set is not a functor mkay
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gistfile1.scala
scala> case class Bad(a: Int) { override def equals(a:Any) = true }
scala> val f = (n:Int) => Bad(n)
scala> val g = (b:Bad) => b.a
...
scala> Set(1,2,3).map(f andThen g)
res2: scala.collection.immutable.Set[Int] = Set(1, 2, 3)
scala> Set(1,2,3).map(f).map(g)
res3: scala.collection.immutable.Set[Int] = Set(1)
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I'm just saying that equals = true breaks https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.21

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